1. Field of the Invention
The present invention relates to a perpendicular magnetic recording medium comprising a magnetic recording layer and a soft magnetic layer and adapted for a high-density magnetic recording and to a magnetic recording-reproducing apparatus using the particular perpendicular magnetic recording medium.
2. Description of the Related Art
In a perpendicular double-layer medium formed by laminating a perpendicular recording layer on a soft magnetic layer having a high permeability, the soft magnetic layer plays the role of a part of the function of the magnetic head such that the recording magnetic field that has been generated from the magnetic head and that has magnetized the perpendicular recording layer is passed in a horizontal direction and be brought back to the side of the magnetic head, thereby improving the recording-reproducing efficiency. On the other hand, the soft magnetic layer tends to form domains for decreasing the magnetostatic energy, and domain walls are formed at the boundaries between the adjacent domains. It is known in the art that a spike-like noise is generated at the site where the domain wall is present. The spike-like noise can be suppressed by suppressing the formation of the domain walls in the soft magnetic layer. Some methods for suppressing the domain wall formation are proposed in, for example, Jpn. Pat. Appln. KOKOKU Publication No. 3-53686 and Jpn. Pat. Appln. KOKAI Publication No. 5-258274. It is proposed that a hard magnetic layer or an antiferromagnetic layer is formed above or below a soft magnetic layer so as to fix the direction of the easy magnetization of the soft magnetic layer to the radial direction for suppressing the formation of the domain wall in the soft magnetic layer.
In view of the control of the direction of the easy magnetization of the soft magnetic layer, it is convenient to use a hard magnetic layer that permits easily controlling the direction of magnetization of the hard magnetic layer itself. Where a hard magnetic layer is formed below the soft magnetic layer, the hard magnetic layer is far from the magnetic head enough so as to inhibit the writing of the information magnetization. However, the hard magnetic layer is basically equal to the longitudinal magnetic recording medium in the material and the magnetic characteristics, and therefore provide a source of medium noise. Also, it is desirable for the soft magnetic layer to be thin in view of the manufacturing cost and the dust generation during the film forming process. However, the noise from the hard magnetic layer is increased if the soft magnetic layer is made thinner and the distance between the hard magnetic layer and the magnetic head is decreased. It follows that the use of a hard magnetic layer generates a problem particular where the soft magnetic layer is made thinner.
On the other hand, the use of an antiferromagnetic layer is advantageous over the use of a hard magnetic layer in view that a noise is not generated because magnetization does not take place. However, since the antiferromagnetic layer does not have a macroscopic magnetization unlike the hard magnetic layer, it is difficult to control directly the direction of the magnetic spin of the antiferromagnetic layer. Therefore, in order to fix the direction of the magnetization of the soft magnetic layer to the radial direction, a soft magnetic layer is formed first, followed by forming an antiferromagnetic layer on the soft magnetic layer with a magnetic field applied in the radial direction, as disclosed in Japanese Patent Publication No. 3-53686 referred to above. However, where an antiferromagnetic layer is laminated on a soft magnetic layer as proposed in the Japanese patent document quoted above, the distance between the soft magnetic layer and the magnetic head is increased because of the presence of the antiferromagnetic layer and therefore give rise to the problem that the recording magnetic field generated from the magnetic head tends to be diffused so as to lower the recording magnetic field intensity and the recording resolution.
As described above, in the conventional perpendicular double-layer medium comprising a soft magnetic layer and a perpendicular recording layer laminated on the soft magnetic layer, the spike-like noise was suppressed by magnetizing the soft magnetic layer in the radial direction by using a hard magnetic layer or an antiferromagnetic layer. However, the use of the hard magnetic layer gives rise to the problem that noise is generated from the hard magnetic layer itself. On the other hand, in the case of using an antiferromagnetic layer, it is difficult to control the direction of the magnetization. Also, an additional problem is located in that the distance between the magnetic head and the soft magnetic layer is increased.